1. Field of the Invention
The present invention relates to flatbed-type image reading devices configured to read an image on an original document placed on a document mounting plate.
2. Description of Related Art
A known flatbed-type image reading device reads an image on an original document with an image reading unit, e.g., an image scanner. Specifically, in the known image reading device, the image reading unit is positioned below a document mounting plate e.g., a glass, and scans a surface of the original document, which is placed on the document mounting plate with the surface facing down. The image reading unit includes a photoelectric converter configured to read an image on an original document by converting light reflected off the original document into an electrical signal. The types of the photoelectric converter include a charge coupled device (“CCD”) or a contact image sensor (“CIS”). The CIS reads an image on an original document in near direct contact with the original document. Therefore, the distance between the CIS and an original document influences an image reading accuracy. The known image reading device is scanned while constantly maintaining the distance between an original document placed on the document mounting plate and the image reading unit, i.e., a lower side of the document mounting plate and the image reading unit.
A known image reading device, e.g., Japanese Laid-Open Patent Publication No. 11-98322, describes an image reading device including a photoelectric converter, e.g., a CIS, driven rotating members, e.g., rollers, and urging devices, e.g., springs. An outer peripheral surface of each driven rotating member protrudes further toward the lower surface of the document mounting plate than the photoelectric converter. The urging devices press the driven rotating members to make the outer peripheral surfaces of the driven rotating members contact the lower surface of the document mounting plate. As the outer peripheral surfaces of the driven rotating members contact the lower surface of the document mounting plate, the distance between photoelectric converter and the lower surface of the document mounting plate is constantly maintained and the image reading unit is moved smoothly by rotating the driven rotating members.
Specifically, in the known image reading device, the CIS is held in a traveling member that has a box shape with an opening end facing upward and that is elongated in a main scanning direction perpendicular to a sub-scanning direction. Rollers are supported in a cantilever manner by end walls of the CIS spaced apart in the longitudinal direction of the CIS. A flange-shaped extended portion is positioned on the end wall of the CIS so as to extend outward in the longitudinal direction of the CIS. The spring is positioned between a lower surface of the extended portion and a bottom plate of the traveling member.
Various positional relations between the rollers and the springs with respect to the CIS may be considered. Referring to a known image reading device shown in FIG. 9A, an urging position B where a spring 102 urges a CIS 103 is placed closer to a central portion of CIP 103 in a longitudinal direction thereof than a contact position A where a roller 101 contacts a lower surface of a document mounting plate 106. More specifically, contact position A is a center point of a portion of roller 101 receiving pressing force by contacting a document mounting plate 106. In the known image recording device, such a force is acted on CIS 103 to raise the central portion of CIS 103 in the longitudinal direction thereof higher than each end of CIS 103. Thus, CIS 103 is convexly deformed, as shown by dashed lines in FIG. 9A. For the purpose of clarity, the deformation is exaggeratedly shown in FIG. 9A. When CIS 103 deforms along the longitudinal direction thereof, distance between CIS 103 and an original document to be scanned varies. Variances in the distance between CIS 103 and the original document adversely affect an image reading or scanning accuracy.
Referring to a known image reading device shown in FIG. 9B, urging position B is placed closer to ends of CIP 103 in a longitudinal direction thereof than contact position A. In this case, such a force is acted on CIS 103 to raise ends of CIS 103 in the longitudinal direction thereof higher than the central potion of CIS 103. Therefore, CIS 103 is concavely deformed, as shown by dashed lines in FIG. 9B. For the purpose of clarity, the deformation is exaggeratedly shown in FIG. 9B. Similarly to the known image reading device shown in FIG. 9A, the image reading or scanning accuracy is adversely affected.
As the length of CIS 103 becomes longer, the influence of the deformation of CIS 103 becomes larger. For example, influences of deformation of CIS 103 is more remarkable when the length of CIS 103 is generally the same as the width, i.e., a shorter side, of a A3-sized original document, than when the length of CIS 103 is generally the same as the width, i.e., a shorter side, of a document with lower width, e.g., an A4-sized original document.
Referring to a known image reading device shown in FIG. 9C, contact position A and urging position B are positioned relatively close to each other. A shaft 104 of roller 101 is supported by CIS 103 in a cantilever manner. Spring 102 urges an extended portion 105 of CIS 103. Upward urging force of spring 102 is acted on a base end D of shaft 104 whereas downward pressing force is acted on roller 101 at contact position A. Therefore, roller 101 may be tilted, as indicated by dashed line in FIG. 9C, and functions of roller 101 may be impaired.